1. Field of the Invention
The present invention relates to an integral type flat antenna provided with a converter function, and in particular relates to an integral type flat antenna provided with a converter function which is mainly designed to receive electromagnetic waves transmitted from satellites. More specifically state, the flat antenna of the present invention is designed to serve as a GPS (Global Positioning System) antenna for receiving electromagnetic waves from GPS satellites, and such an antenna is particularly used for car navigation systems.
2. Description of the Prior Art
Various types of integral type flat antennas provided with converter functions are known in the prior art, and one of these antennas for GPS use is shown in FIG. 1.
In this regard, FIG. 1 is a cross-sectional view of a conventional integral type GPS antenna provided with a converter function. As shown in this drawing, the GPS antenna is basically constructed from an antenna element 1, an antenna substrate 5 which supports the antenna element 1 thereon, and a housing 12 made from a steel plate which is provided below the antenna substrate 5 to support the antenna substrate 5.
The antenna element 1 includes a dielectric portion 1a made from a dielectric substance such as ceramic or the like and a feeding point 2 provided roughly in the center of the top surface of the dielectric portion 1a. Now, because the surrounding environment can give rise to the formation of static charges to the dielectric portion 1a, the antenna element 1 needs to be grounded in order to stabilize its characteristics. To accomplish this, grounding planes 3, 4 made of copper foil are provided on the top and bottom surfaces of the antenna substrate 5. Further, grounding is established by mounting the antenna element 1 in the center of the top surface of the grounding plane 3 which is provided on the top surface of the antenna substrate 5 and by connecting the grounding plane 3 to an earth via an outer conductor of a coaxial cable 20 (described below).
Further, a terminal portion 6 extends downwards from the feeding point 2 of the antenna element 1 through the inside of the antenna element 1 and through a through-hole 7 formed in the antenna substrate 5 so as to protrude below the bottom surface of the antenna substrate 5. The protruding portion of the terminal portion 6 is soldered to the antenna substrate 5 at a soldering portion 8. Further, a receptacle 9 is provided below the antenna substrate 5 in the vicinity of the through-hole 7, and the receptacle 9 1s connected to the soldering portion 8 via a circuit pattern 10.
Furthermore, the grounding plane 4 provided below the antenna substrate 5 has cut-out portions around the border of the soldering portion 8, circuit pattern 10 and receptacle 9, and thereby it is electrically insulated from these elements. Further, the grounding plane 3 provided on the top of the antenna substrate 5 is also electrically insulated from the terminal portion 6.
Further, positioning apertures 11, 11 are formed in the antenna substrate 5, and bosses 13, 13 are erected on the upper surface of an upper case 12a of a housing 12 at positions corresponding to the positioning apertures 11, 11. In this way, the antenna substrate 5 equipped with the antenna element 1 is mounted onto the upper case 12a by fitting the positioning apertures 11, 11 over the bosses 13, 13.
Provided inside the upper ease 12a is a front end substrate 17 on which a frequency conversion circuit 16 is mounted. The frequency conversion circuit 16 is constructed by mounting electrical parts 15, 15, such as integrated circuits, oscillators and the like, onto the bottom surface of a double-sided substrate 14. Further, square-shaped apertures 18, 19 are formed roughly in the center of the upper case 12a and the front end substrate 17, respectively, at positions which correspond to the receptacle 9. Further, one end of the inner conductor of the coaxial cable 20, which serves as a feeding line having a predetermined impedance (e.g., 50 .OMEGA.), is connected to the receptacle 9 and the other end of the inner conductor is connected to the frequency conversion circuit 16 of the front end substrate 17 via the square-shaped apertures 18, 19. In this regard, as was explained above, in order to ground the antenna element 1, one end of the outer conductor of the coaxial cable 20 is connected to the grounding plane 4 and the other end thereof is connected to the housing 12 and the like.
Now, because GPS electromagnetic waves and the like transmitted from satellites are generally high frequency waves in the gigahertz range of 3-30 GHz, the signal characteristics can easily be deteriorated when electric signals based on such received electromagnetic waves are processed in the frequency conversion circuit 16. For this reason, in order to prevent such deterioration in signal characteristics, it is preferred that the frequency conversion circuit 16 has a circuit design in which signals flow as linear as possible.
Thus, in such prior art GPS antenna, the coaxial cable 20 bends roughly 90 degrees after passing through the square-shaped apertures 18, 19 and then runs parallel to the underside surface of the front end substrate 17 until it reaches a position near one end of the front end substrate 17 (shown in the drawing as the right end). At that position, the coaxial cable 20 is connected to a receptacle 21 which 1s provided on the front end substrate 17 to act as a signal input portion. Further, the circuitry is designed such that the signals which are inputted at the input portion of the frequency conversion circuit 16 (i.e., the receptacle 21) flow roughly linearly toward the other end of the front end substrate 17 (shown in the drawing as the left end) and reach an output connector 22 provided at the other end of the front end substrate 17 to act as an output portion.
Now, because this type of GPS antenna is mainly used for car navigation system, it is generally fixed to the top surface of a car's trunk or the like. For this reason, it is preferred that the antenna be made as thin as possible.
However, as described above, in such prior art integral type GPS antenna provided with a conversion function, two substrates, namely the antenna substrate 5 and the frequency conversion circuit substrate 17 (i.e., the front end substrate 17) must be provided separately, and only the front end substrate 17 is housed inside the housing 12. Further, because the coaxial cable 20 for connecting the feeding point 2 of the antenna element 1 and the frequency conversion circuit 16 must run below the front end substrate 17 up to the signal input portion (receptacle) 21, a prescribed space must be provided below the front end substrate 17. For these reasons, the housing must have a specific height, and in addition it is also necessary for the antenna to have a certain height for mounting the antenna substrate 5 above the housing 12. Therefore, it is very difficult to construct a thinner-type flat antenna.
Moreover, because such prior art antenna requires two substrates that must be manufactured separately as well as the coaxial cable, there is an increase in the number of parts and the number of manufacturing steps. Such structure leads to a complex manufacturing process, and it also leads to high manufacturing costs due to the relatively expensive price of coaxial cables.
Furthermore, when used for car navigation, it is preferred that such GPS antennas be constructed so as to be resistant to vibrations transmitted from the car. However, in the structure described above, it is easy for the antenna to be affected by such vibrations because the antenna element 1 and the antenna substrate 5 (and the grounding planes 3, 4) are located outside the housing 12 and are supported by bosses. As a result, the electrical connections such as the soldering portion 8 are liable to suffer damage due to vibrations. Further, because the connections of the coaxial cable 20 are carried out by means of the receptacles 9, 21, vibrations from the car can cause the coaxial cable 20 to become loosen or fallen out from the receptacles, thereby giving rise to poor or broken connections.
Furthermore, since these types of flat antennas are usually used outdoors, it is desired that such antennas are designed so as to be able to adequately withstand environmental conditions such as rain, snow, heat and the like.